The alu sequence appears to have arisen by partial duplication of the
sequence
B1 which is still present in bovine and rodent genomes. That B1 sequence
appears to have arisen from genomic 7S RNA. Where the short poly A tail
came
from is not clear-because there is an abundence of possibilities.
Alu has no ORFs and any enzyme action would have to be ribozymal. To my
knowledge, no one has looked for ribozymal action in alu, but there are
somewhat similar sequences in the frog genomes which do. It is currently
held
that alu moves by retrotranscription with reverse transcriptase being
supplied
in trans by LINE-1 elements.
The evolution of LINE-1 is most probably from a retrovirus. There is an
instructive series of IAP sequences in mice, which starts with a
obviously retroviral sequence including an env gene. At the other end of
this
sequence there is a sequence which is structurally closely related to
LINE-1.
In addition of course there are a number of endogenous retrovirus (HERVs)
sequences which I expect some day someone will show that they are up to no
good.
The question of the origin of the retroviruses is not so clear. The degree
of
homology between e.g reverse transcriptase and other polymerases indicates
a
common ancestor, although any attempt to put a time frame to it will only
be a
wild guess. The question is further complicated by the fact that separate
parts of the virus genome may have arisen quite independently.
In common with many other people. I don't like extra terrestrial origin
stories because it makes the story unneccesarily complicated without
explaining anything. This is the principal of parsimony.
If you want fruther details try an on line search. There are a number of
excellent reviews available and some of the keywords from above may help.
Chris Driver
Deakinj University
Australia
Chris Driver, Ph D
School of Biology and Chemistry, Rusden Campus
Deakin University
662 Blackburn Rd
Clayton, VIC, 3168
AUSTRALIA